I'm curious because...I ran 87 octane in the car once by mistake when I first got it, and then checked the timing to discover a reading of 20 degrees of base timing. Never heard a single ping. I was like "WHAT THE *****".... 
The harmonic balancer was shot and losing rubber so I don't know if it was off, but I can tell you that I backed the timing off to 10 degrees and it didn't run much different, just got a little more sluggish...and it started easier. Now I did have 4 broken fuel injectors then and a host of bad sensors causing the car to run ice cold and extremely rich... I have new injectors and all that now and a new balancer and the timing is holding at a steady 13.
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'91 Mustang GT 5.0 AOD Daily Driver
Current stats: NEW 302 short block, E-303 cam (@.050, 220/220, .498/.498, 110 degrees), 1.6 roller rockers, Performer RPM heads (aluminum, 1.9/1.6, 60cc chambers), Cobra intake, Granatelli 75mm MAF, Billet AFPR, Equal-Length Shorty Headers, Off-Road X Pipe, Flowmasters, 4.10 POSI
13.73 at 103.8, ~2.2 60'
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